Chrominance channel circuits



Aug 9 1955 A. L. HAMMOND CHROMINANCE CHANNEL CIRCUITS Filed Jan. 18,1954 maOomOmQS OP United States Patent C CrmornNmsCn CHANNEL CmCUrrsArthur L. Hammond, Los Angeles, Calif., assignor to Hoffman RadioCorporation, a corporation of California Application January 1S, 1954,Serial No. 404,545

4 Claims. (Cl. VIS- 5.4)

This invention is related to color television receiving apparatus andmore particularly to an improved chrominance channel circuit foremployment in such apparatus.

In the past, many attempts have been made to design a satisfactorychrominance channel circuit to detect and prepare the chrominancesignals for matrixiug and subsequent routing to the image reproducingdevice or videoscope. invariably certain problems are encountered whichrender chrominance channel circuits presently in use decient in somerespect. The principal diihculty lies in the fact that the conventionalband-pass filters employed in such chrominance circuits introduce unqualphase shifts in the chroma signals being received, and, indeed, adifferent delay or advance in phase for each separate side bandfrequency. This undesirable phenomenon prevents synchronization of thechroma signals with the black-and-white signals at the videoscope, and,hence, color indelity.

Therefore, it is an object of this invention to provide an improvedchrominance channel circuit for employment in color television receivingapparatus.

It is a further object of this invention to provide an improvedchrominance channel circuit having means to compensate for phase shiftsexperienced by the chroma signals upon their passage through thechrominance channel circuit.

According to this invention the chroma subcarrier and associatedsidebands are selected by a first bandpass filter and fed to twomodulators to be intermodulated with a signal having a frequency at thesecond harmonic of the subcarrier, and the lower sideband of each of themodulator output signals is selected by a second band-pass lter, similarto the first, and fed to a feed-back amplifier which in turn is coupledto the receiver matrxing system.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The presentinvention, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may best be understood byreference to the following description, taken in connection with theaccompanying drawing, in which:

The sole figure is a diagram of a chrominance channel circuit accordingto this invention.

In the subject figure, the output signal of the video intermediatefrequency amplifier strip (not shown) passes through a second detectorstage 10 to a video amplifier stage 11 having two output circuits. The'r'st output circuit, constituting the luminance channel circuit, isdirectly connected to the inverter and matrixing circuits 12, 13, and14; and the second output circuit is connected through chrominancechannel circuit 15 to the inverter and matrixing circuits 13 and 14.

'Chromin'ance channel circuit 1S consists of band-pass filter 16 whichselectively passes the chroma subcarrier and side-bands to two modulatorstages 17 and 18, to

i' channel frequencies.

2,715,156 Patented Aug. 9, 1955 'ice be intermodulated with the outputsignal from frequency doubler stage 20 which is excited by a signal atthe chroma subcarrier frequency obtained from oscillator 19. The outputsignal of modulator stage 17 is fed through band-pass filter 21 to the Idemodulator stage 22, to be intermodulated with the output signal oflocal oscillator stage 19. The output signal of modulator stage 18 isfed through band-pass filter 23 to the Q demodulator stage 24, to beintermodulated with the output signal of oscillator stage 19 after suchsignal is passed through a quadrature amplifier stage 25. Bandpass lters21 and 23 are chosen to be identical both as to phase and frequencycharacteristics with band-pass filter 16 and, hence, likewise pass onlythe chrominance The output signal from l demodulator stage 22 is fedthrough feedback amplifier stage 26, which acts as an l channelband-pass llter, to inverter and matrix circuit 13. The output signalfrom Q demodulator stage 24 is fed through feedback amplier stage 27,which acts as a Q channel bandpass filter, to inverter and matrixcircuit 14. Inverter and matrix circuits 12, 13, and 14 are subsequentlycoupled to the videoscope (not shown).

The circuit of the subject ligure operates as follows: Band-pass lter 16selects the chroma subcarrier and sidebands for passage throughchrominance channel `circuit 15.

A particular sideband frequency present in the chrominance channeloutput signal of video amplifier stage 11 may be represented by theequation:

(1) ES=AS cos wt where Es is the particular sideband frequency signal,As is the maximum signal amplitude, w is two-pi times the signalfrequency, Iand t equals time. As this signal Es is impressed upon andpassed through band-pass lter 16 it will experience a certain phaseshift, owing to the properties of lter 16. Filter 16 may be so designedthat the chroma subcarrier, itself, will experience no change in phasewhile passing through it. However, frequencies of the lower sidebandwill experience a phase advance with respect to the subcarrier, andfrequencies of the upper sideband will experience a phase delay withrespect to the subcarrier, Selecting a particular lower sideband signalE1, it will be found that .E1 will .have advanced in phase by an angle 0by Virtue of its passing through filter v16.

Then,

where A1 is the maximum amplitude of E1. This phase shift of 0 willdisturb synchronization of the chroma signals with the monochromesignals of the luminance channel at the videoscope. Hence, it isdesirable to correct this phase error.

The effect when sideband signal E1 is intermodulated with a harmonic ofthe subcarrier, designating the particular order of harmonic by K, i.e., K equals l for the first harmonic (fundamental), K equals 2 for thesecond harmonic (twice the fundamental frequency), etc., should now bestudied. The heterodyning harmonic signal may be designated as E2, and

(3) E2=Az cos Kwt where A2 is the maximum amplitude, w is two-pi timesthe subcanier frequency, and t again is time. lt is recalled thatamplitude modulation is produced through varying the amplitude of thewave to be modulated, the deviation of the amplitude from theunmodulated value being directly proportional to the instantaneous valueof the modulating wave but independent of its frequency.

O ln accordance with this definition, the amplitude of the modulatedwave is a function of time given by where ka is a constant ofproportionality; hence the output heterodyned signal may be defined YEa=A(t) cos Kwt and from (4) En=(A1-lkaA2 cos (WH-0)) cos Kwt (5)Eo=A1(l-l-m cos (WH-0)) cos Kwt where m, the modulation factor, equalskaAz Ai .41% eos ((K-rwi-o) which is to say Eo=a carrier-I-an uppersideband-l-a lower sideband By letting K equal 2, i. e., by using as aheterodyning signal the output of stage which is at double thechrominance subcarrier frequency, and selecting the lower sideband it isfound that this lower sideband signal corresponds to the originalsideband signal of Equation 2 with the exception that the new sidebandsignal lags the original chroma subcarrier by au angle 6. Thus, byemploying band-pass filters 21 and 23, which are duplicates of band-passfilter 16, we nd that while the original lower sideband signals (A coswt) experiences a phase advance of 9 upon passage through chrominanceband-pass filter 16, this resultant signal (A cos (WH-0)) willexperience a phase delay of 20 when passing through each `of the twomodulator stages 17 and 18 (see the third term of Equation 7), and asubsequent phase advance of 0 upon passage through phase compensatingband-pass filters 21 `and 23; hence, restoration to the phase of theoriginal side-band signal is nally achieved.

It is accordingly seen that all frequencies constituting y the upper andlower sidebands of the chroma subcarrier will be restored in phase totheir original relationships by virtue of the phase-shift compensatingfeature of the Vchrominance channel according to this invention.

0f course, any harmonic of the chroma subcarrier may be employed byusing the appropriate frequency multiplier for stage 20. But then afrequency multiplier must also be inserted between the local oscillatorand the I and Q demodulators, this latter frequency multiplier having anorder of multiplication equal to K-l, where K is the multiplicationfactor of frequency multiplier stage 20.

Absence of phase shifts in the chroma signals is further assuredby theemployment of feedback amplifier stages 26 and 27, instead of theconventional low-pass lters, to limit the I and Q channels to thedesired bandwidths. Feedback amplifiers are characterized by no timedelay and very constant phase relations over a band of frequencies,whereas low-pass filters are characterized by a phase or time delay ofapproximately one microsecond at the frequencies used in the chrominancechannel, thus requiring a compensating one microsecond delay line in theluminance channel. This requirement is thus avoided by use of feedbackamplifiers.

From the foregoing description of the present invention, it is seen thatthere has beenprovided for use in color television receivers a new anduseful chrominance channel circuit which reduces to a minimum phasedistortion of chroma signals occurring in the chrominance channel, andconsequently improves the overall synchronization of the color impulseswith the brightness impulses at the videoscope.

While this invention has been described in terms of color televisionreceiving apparatus, this invention may also be applicable in otherinstances Where it is desired to retain certain phase relationships.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects, and therefore, the aim in the appendedclaims is to cover all such changes and modifications as fall within thetrue spirit and scope of this invention.

I claim:

1. A phase restorer circuit for employment in the I and Q demodulatorbranches of the chrominance channel of a color television receiver torestore the phase relationships between the chroma sidebands and thechroma subcarrier Vto those existing immediately prior to the selectionof such sidebands and carrier by a chrominance channel selective filter,said phase restorer circuit including, in combination, a modulator stagehaving two input terminals and an output terminal, a phase restorerfilter having sideband phase shift characteristics correspondingvtothose of said chrominance channel selective filter, a local oscillator,and a frequency multiplier, said local oscillator being coupled throughsaid frequency multiplier toone of said input terminals of saidmodulator, the remaining input terminal of said modulatory being adaptedfor coupling to said chrominance channel selective filter, and saidphase restorer filter being coupled to` said output terminal of saidmodulator.

2. A phase compensating band-pass filter circuit for employment in thechrominance channel of a color television receiver including, incombination, a rst bandpass filter, a first modulator having a firstinput circuit, a second'input circuit, and an output circuit, and asecond modulator having a first input circuit, a second input circuit,and an output circuit, said first band-pass filter being coupled to saidfirst input circuit of said first modulator and to said first inputcircuit of said second modulator; a local oscillator and a frequencymultiplier, said local oscillator being coupled through said Vfrequency*multiplier to said second input circuit of said first modulator and tosaid second input circuit of said second modulator; a second band-passfilter and a third band-pass filter each having sideband phase shiftcharacteristics corresponding to said first band-pass filter, saidoutput circuit of said first modulator being coupled to said bandpassfilter, and said output circuit of said second modulator being coupledto said third band-pass filter.

3. A chrominance channel circuitfor employment in color televisionreceivers and comprising, in combination,

a first band-pass filter, a first modulator stage having a first inputcircuit, a second input circuit, and an outputV circuit, and a secondmodulator stage having a first input circuit, a second input circuit,and an output circuit, said first band-pass filter being connected tosaid first input circuit of said rst modulator stage and to said firstinput circuit of said second modulator stage; a local oscillator stageand a frequency doubler stage, said local oscillator'v stage beingconnected through said frequency doubler stage to said second inputcircuit of said rst modulator stage and to said secondV input circuit ofsaid second modu-A lator stage; a second band-pass filter havingcharacteristics corresponding to said first band-pass filter and an Idemodulator stage having a rst input circuit, a second input circuit,and an output circuit, said output circuit of said first modulator stagebeing connected through said second band-pass filter to said firstinput' circuit of said l demodulator stage, and said local oscillatorstage being connected to said second input circuit of said I demodulatorstage; a third band-pass iilter having characteristics corresponding tosaid rst band-pass filter, a Q demodulator stage having a iirst inputcircuit, a second input circuit, and an output circuit, and a quadratureamplifier stage, said local oscillator stage being connected throughsaid quadrature amplifier stage to said second input circuit of said Qdemodulator stage; a rst feedback amplifier stage, and a secondfeed-back amplifier stage, said output circuit of said I demodulatorstage being connected to said first feed-back amplifier stage, and saidoutput circuit of said Q demodulator stage being connected to saidsecond feed-back amplier stage.

4. A chrominance channel circuit for employment in color televisionreceivers and comprising, in combination, a first band-pass lter, a irstmodulator stage having a rst input circuit, a second input circuit, andan output circuit, and a second modulator stage having a rst inputcircuit, a second input circuit, and an output circuit, said rstband-pass lter being connected to said iirst input circuit of said rstmodulator stage and to said rst input circuit of said second modulatorstage; a local oscillator stage and a rst frequency multiplier stage,said local oscillator stage being connected through said rst frequencymultiplier stage to said second input circuit of said rst modulatorstage and to said second input circuit of said second modulator stage; asecond frequency multiplier stage, a second band-pass filter havingcharacteristics corresponding to said rst band-pass filter, and an Idemodulator stage having a first input circuit, a second input circuit,and an output circuit, said output circuit of said first modulator stagebeing connected through said second band-pass filter to said rst inputcircuit of said I demodulator stage, and said local oscillator stagebeing connected through said second frequency multiplier stage to saidsecond input circuit of said I demodulator stage; a third band-pass lterhaving characteristics corresponding to said first band-pass lter, a Qdemodulator stage having a rst input circuit, a second input circuit,and an output circuit, and a quadrature amplifier stage, said localoscillator stage being connected through said second frequencymultiplier stage and said quadrature amplifier stage to said secondinput circuit of said Q demodulator stage; a first feed-back amplifierstage, and a second feed-back amplifier stage, said output circuit ofsaid I demodulator stage being connected to said rst feed-back amplifierstage, and said output circuit of said Q demodulator stage beingconnected to said second feed-back amplifier stage.

References Cited in the le of this patent UNITED STATES PATENTS2,667,534 Creamer Jan. 26, 1954 2,668,189 Reddeck Feb. 2, 1954 2,674,651Creamer Apr. 6, 1954

